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Embryonic Screening Selecting for the Perfect Child

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Embryonic Screening Selecting for the Perfect Child Focus: Brave New World EMBRYONIC SCREENING SELECTING FOR THE PERFECT CHILD By Peter Cai reimplantation genetic diagnosis Development of PGD “P(PGD), also known as embry- The development of embryonic onic screening, allows parents and screening began in 1967 with Edwards doctors to screen fertilized embryos and Gardner’s ability to determine the and pre-fertilized oocytes for numerous gender of live rabbit blastocysts while characteristics, particularly genetically retaining their viability (1). Their ability “However, the inherited diseases. PGD is an extension to harvest embryos and identify future development of PGD of in vitro fertilization (IVF). In IVF, traits without harming the embryos set opens up a Pandora’s oocytes are obtained, fertilized outside the stage for application of PGD in of the body, and implanted in the humans. In 1989, Coutelle et al. estab- Box of opportunities woman’s uterus. While IVF facilitates lished a preimplantation diagnostic test to dictate a child’s fertilization of eggs and implantation for cystic fibrosis for human embryos future traits. ” of embryos, PGD goes a step further (2), and in 1992, Handyside et al.’s IVF than simply assisting couples to become and PGD testing led to the successful pregnant. It screens these embryos and birth of a healthy, cystic-fibrosis-free selects only those with desired traits to girl (3). be implanted and eventually grow into Throughout the past 2 decades, a baby: scientists look for genetic flaws PGD has made rapid progress as the and actively select embryos for implan- development of efficient PCR tech- tation that are disease-free. niques has allowed for more varied, However, the development of PGD accurate, and cheaper diagnoses. Since opens up a Pandora’s Box of opportu- Handyside’s first PGD baby, the list of nities to dictate a child’s future traits. genetic conditions which can be tested As PGD becomes more widespread is rapidly expanding; the UK’s Human and successful, it presents a moral Fertilisation and Embryology Authority dilemma: where is the line between currently lists over 60 conditions that creating a healthy child and trying to are testable by PGD (4). mold a “perfect” child? credit: www.cdc.gov fall 2008 • Harvard Science Review 21 cai.indd 21 2/9/2009 11:23:45 PM Focus: Brave New World The top picture shows a polar body biopsy, which occurs 14-20 hours after normal fertilization. The bottom pic- ture depicts cleavage-stage biopsy, which happens about 72 hours after fertilization has occurred. method (used in results in one large oocyte and two 94% of embryon- smaller polar body byproducts. (Figure ic screens) is the needed here). Since the polar bodies are “cleavage-stage waste products of oogenesis and not biopsy” where 1 needed for embryo development, polar or 2 cells from a body (PB) biopsies provide a very safe blastomere (8 cell alternative to taking biopsies of devel- or later develop- oping embryos. First used by Verlinsky mental stage) are (7), a polar body biopsy bypasses the sectioned out for ethical concern regarding the status of analysis. The ob- embryos as living beings, which is why vious advantage it is used in countries such as Germany to this method where cleave-stage embryo selection is that it allows is banned. Unfortunately, this biopsy analysis of the method can be misleading because it DNA contrib- only provides information on mater- uted by both the nally transmitted diseases (the egg has mother and the yet to be fertilized and does not contain father, since the DNA from the sperm) and genetic in- cells were taken formation may be degraded, since polar The science behind PGD from a fertilized embryo. Nonetheless, bodies are effectively “junk.” However, Embryonic screening has 3 main there are several scientific and moral the ethical and safety benefits of PB components: obtaining an embryo, cons to this method. biopsies have encouraged reproductive taking a biopsy of it, and completing Scientifically, this sectioning disrupts researchers to continue to develop reli- a genetic analysis of the biopsy. The development and can cause the biop- able PB-based diagnoses (8). procedure for obtaining embryos for sied cell to develop abnormally, defeat- The last biopsy method, “blastocyst PGD is identical to that used in IVF. ing the purpose of the screen. Also, biopsy,” presents many of the moral Controlled ovarian stimulation uses a chromosomal mosaicism is common challenges that cleavage stage biopsy set of timed hormone supplements to at this stage, which can make results of presents, but overcomes the limited tis- stimulate production of surplus oocytes the tests irrelevant (5). Cleavage stage sue problem by taking a biopsy later in for harvesting. Finally, to fertilize the biopsies obtain far less tissue than the development (figure of human embryo oocyte and create an embryo, sperm is other two techniques, forcing scientists Nature Reviews Genetics 3, 941-955 (December 2002) development). While the additional tis- directly injected into the oocyte, a pro- to rely on diagnostic techniques that sue may lead to more accurate genetic cedure called intracytoplasmic sperm present their own problems. Although diagnoses, since blastocysts are later in injection (5). it is a preferable method from a scien- development, fewer are available for One of the most controversial pro- tific perspective, it has been rejected on screening. Furthermore, the delaying cedural components of PGD is the a moral basis by individuals or countries the biopsy limits the time before im- biopsy. Depending on the personal, that consider these embryos to be living plantation and the time for diagnosis religious, and legal definitions of which and thus forbidden to probing, selec- (9). embryonic stage constitutes a living be- tion, and disposal (6). ing, the following three biopsy options The second available method is called Genetic Screening present varying scientific and moral a “polar body biopsy.” Oocytes develop There are a number of screens that trade-offs. via a two-step meiotic oogenesis, during can be used during PGD to detect ge- The first and most popular biopsy which uneven division of cytoplasm netic abnormalilties. Fluorescent in situ credit: Braude, et al. “Preimplantation genetics diagnosis.” credit: Braude, et al. “Preimplantation 22 Harvard Science Review • fall 2008 cai.indd 22 2/9/2009 11:23:45 PM Focus: Brave New World ing may one day go beyond simply helping parents bear healthy children and instead be used for sex selection and choosing specific traits (13). These possibilities of selecting embryos for ideal children is not far from the limits of current science and threatens to eliminate the much beloved surprise of having children. —Peter Cai ’10 is a Molecular Cellular Biology concentrator in Adams House. A four-cell embryo has undergone fluorescence in situ hybridization with probes at chromosomes 13, 16, 18, 21 and 22. In the original color picture, each of the probes would be a different color. References (1) Edwards RG, Gardner RL. Sexing of live rabbit blastocysts. Nature (May 1967) 214: 576-577. hybridization (FISH) probes chromo- Despite all these challenges, embry- (2) Coutelle C, Williams C, Handyside A, Hardy K, somes with fluorescent DNA probes onic screening has been hailed a success Winston R, Williamson R. Genetic analysis of DNA from single human oocytes: a model for preimplan- specific for various chromosomal for helping otherwise infertile couples tation diagnosis of cystic fibrosis. BMJ (July 1989) 299: 22-24. segments and is particularly useful for bear healthy children. Out of 4748 PD (3) Handyside AH, Lesko JG, Tarín JJ, Winston RM, detecting abnormal chromosome ar- attempts, 754 babies have been born, Hughes MR. Birth of a normal girl after in vitro fer- tilization and preimplantation diagnostic testing for rangements such as trisomy 21 which and since screening eliminates embryos cystic fibrosis. NEJM (Sept 1992) 327: 905-909. causes Down’s syndrome (10). carrying chromosomal abnormalities (4) UK HEFA. http://www.hfea.gov.uk/en/910.html (5) Preimplantation diagnosis. Delhanty JD. Prenat To detect for more specific genetic that would likely abort a pregnancy, Diagn (Dec 1994) 14: 1217-27. abnormalities, the polymerase chain “The overall pregnancy rate per transfer (6) Fasouliotis, JS, and Schenker, JG. “Preimplanta- tion genetic diagnosis principles and ethics.” Human reaction (PCR) amplifies a cells’ lim- is 23.3%, much higher than the rate in Reproduction, Vol. 13 No. 8, p 2238-2245, 1998. (7) Verlinsky Y, Ginsberg N, Lifchez A, Valle J, ited amount of genetic material to an IVF patients in the comparable age Moise J, Strom CM. Analysis of the first polar body: amount sufficient for further testing group (average age, >39 years) without preconception genetic diagnosis. (Oct 1990) Human Reproduction 5: 826-829. (10). While traditionally a very robust PGD.” For couples using IVF, PGD led (8) Altarescu G, Brooks B, Eldar-Geva T, Margal- assay, the use of PCR in PGD exac- to a fourfold reduction of spontaneous ioth EJ, Singer A, Levy-Lahad E, Renbaum P. Polar Body-Based Preimplantation Genetic Diagnosis for erbates the traditional problems of abortion (11). N-Acetylglutamate Synthase Deficiency. Fetal Diagn PCR due to the extremely low amount Ther (August 2008) 24: 170-176. (9) Ogilvie, C.M., et al. “Preimplantation Genetic of template available in PGD (in the Conclusion Diagnosis.” Journal of Histochemistry and Cytochem- istry. Vol 53(3): 255-260, 2005. case of a cleavage stage biopsy, a While these scientific successes (10) Dreesen J, Drüsedau M, Smeets H, de Die- single strand). The high number of validate embryonic screening as a nec- Smulders C, Coonen E, Dumoulin J, Gielen M, Evers J, Herbergs J, Geraedts J. Validation of Preimplan- Nature Reviews Genetics 3, 941-955 (December 2002) PCR cycles required makes amplifica- essary assistive reproductive technol- tation Genetic Diagnosis by PCR analysis: genotype tion of contaminants and misleading ogy, controversy still lurks nearby.
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